This invention relates to a drive circuit for driving a plurality of elements to be driven which is furnished with a test circuit for testing itself. The invention also relates to a method of testing the drive circuit.
To fabricate the recording head of a thermal ink-jet printer, for instance, a drive circuit is formed on a semiconductor substrate (silicon substrate) and overlaid on it with a heater (heat-generating resistor) material, followed by the formation of cavities and orifice nozzles on the heater. The cavities serve as pixel-based ink chambers and ink is projected through the orifice nozzles. Since the ink will be supplied from a back side of the silicon substrate, ink supply holes are made through the silicon substrate.
The heater material is usually polysilicon and other materials that are manufactured in the ordinary semiconductor fabrication process. Occasionally, for instance, Ta alloys and other special materials that are not very common in the ordinary semiconductor fabrication process are employed. In a case of using a special material on the heater, a semiconductor fabrication process for forming the drive circuit is followed by another fabrication process that is intended to form a heater using the Ta alloy.
After the semiconductor fabrication process, it is preferable that the semiconductor device can be tested to confirm its completion without connecting the heater as elements to be driven. If the semiconductor device is found inoperable at this stage, for instance, one can avoid waste by not forming a heater on a defective device. However, the conventional drive circuit has had a problem of inability to test the semiconductor device unless the heater is formed on it.
This problem is further described below with reference to two examples of the prior art drive circuit.
FIG. 3A shows in conceptual form a thermal ink-jet head using an exemplary prior art drive circuit in its completed form, and FIG. 3B shows the same thermal ink-jet head except that the drive circuit is in an incompleted form. In a recording head 38 shown in FIGS. 3A and 3B a drive circuit 40 comprises driver transistors T11-T15 and a control circuit 36 for performing on-off control on these driver transistors T11-T15. Driver transistors T11-T15 correspond to heat-generating resistors R11-R15, respectively, which in turn correspond to a plurality of recording elements that perform printing.
The heat-generating resistors R11-R15 are connected at one end to a common ground GND and connected at the other end to sources of the driver transistors T11-T15. Drains of the driver transistors T11-T15 are connected to a common power supply VD1 and their gates are supplied with control signals from the control circuit 36 respectively. In the illustrated case, only five recording elements are shown to simplify the explanation.
The control circuit 36 in the illustrated case is a decoder which comprises decoder circuits D1-D5 for decoding signals C1-C4. The control circuit 36 may be composed of a shift register and other circuits, for instance, as shown in FIGS. 4A and 4B. The control circuit 36 in FIGS. 3A and 3n is replaced by the shift register 42 in FIG. 4A which shows the drive circuit of a thermal ink-jet head in its completed form and in FIG. 4B showing the same drive circuit in an incompleted form.
In addition to the shift register 42 for accessing heat-generating resistors R11-R15, the drive circuit shown in FIGS. 4A and 43 also has a control signal line CTRL2 for performing on-off control of the heat-generating resistors R11-R15, as well as transistors T31-T35 which are switching elements connected to the control signal line CTRL2 to control connection and non-connection between outputs of the shift register 42 and driver transistors T11-T15.
In the recording head 38 in completed form which is shown in FIG. 3A, for instance, when driver transistors T11-T15 are turned on in response to control signals from the control circuit 36, current flows through the respectively corresponding heat-generating resistors R11-R15 and a specified amount of ink is projected from nozzles which are the corresponding recording elements. Conversely, if the driver transistors T11-T15 are turned off, no current flows to the heat-generating resistors R11-R15 and no ink is projected from the nozzles.
In the recording head 38 in an incompleted form which is shown in FIG. 3B, the heat-generating resistors R11-R15 to be driven are not connected to the drive circuit 40 and the sources of the driver transistors T11-T15 are opened, so it is impossible to check if the drive circuit 40 is operative or not. While the problem with the prior art has been described above with particular reference to the recording head of an ink-jet printer, it should be noted that the same problem exists in all other devices that are furnished with the drive circuit, as exemplified by an LED (light-emitting diode) array, an EL (electroluminescent) panel, a magnetostrictor and other devices.
An object of the present invention is to solve above described problem based on the prior art to provide a drive circuit that can be tested for reliability after it has been formed on a substrate by the first fabrication process but before a plurality of elements to be driven are formed on the same substrate by the second fabrication process.
Another object of the present invention is to provide a method of testing the drive circuit.
The object of the invention can be attained by a drive circuit that is formed on a substrate by a first fabrication process and that drives a plurality of elements to be driven being formed on the substrate by a second fabrication process which is different from the first fabrication process, comprising driver transistors provided between a first signal line and the plurality of elements to be driven; a control circuit for performing on-off control on the driver transistors; and a test circuit for testing the drive circuit, the test circuit comprising switching elements and resistor elements that are series connected between a second signal line and junctions of the driver transistors and the elements to be driven respectively, wherein the test circuit performs on-off control on the switching elements in response to a control signal.
Preferably, the plurality of elements to be driven are heat-generating resistors, magnetostrictors or light emitters and are connected between each of the driver transistors corresponding to each of the plurality of elements to be driven and the second signal line.
Another object of the invention can be attained by a method of testing a drive circuit that drives a plurality of elements to be driven, the drive circuit comprising a driver transistors provided between a first signal line and the plurality of elements to be driven, a control circuit for performing on-off control on the driver transistors, and a test circuit for testing the drive circuit, the test circuit comprising switching elements and resistor elements that are series connected between a second signal line and junctions of the driver transistors and the elements to be driven respectively, the test circuit performing on-off control on the switching elements in response to a control signal, comprising steps of: forming the drive circuit on a substrate by a first fabrication process; before the plurality of elements to be driven are formed by a second fabrication process which is different from the first fabrication process, on the substrate on which the drive circuit has been formed, turning on the switching elements of the test circuit in response to the control signal; and performing on-off control on the driver transistors by the control circuit.
Preferably, the drive circuit is tested by changing electric potential of the first signal line to either power supply potential or ground potential.
Preferably, the drive circuit is tested by adjusting electric potential of the first signal line to an intermediate level between power supply potential and ground potential.